1. Field of the Invention
The present invention relates to a current position calculating system, which is installed on a mobile object such as a vehicle or the like, for measuring a travelled distance, a headed direction etc., and for calculating a current position of the vehicle according to the measured values. More particularly, it relates to techniques for reducing an error in measuring the travelled distance.
2. Description of Related Art
A current position of a vehicle is usually calculated on the basis of a headed direction of the vehicle, measured by a direction sensor, such as a gyrocompass, and a travelled distance of the vehicle measured by a vehicle speed sensor or a distance sensor.
Generally, a travelled distance of a vehicle is obtained by measuring the number of revolutions of a transmission output shaft or a tire, and multiplying the result by a distance factor, which corresponds to a distance the vehicle travelled for each revolution of the tire.
In addition, there is known a technique in which in order to correct an error in the current position obtained from the headed direction and the travelled distance as described above, the obtained current position of a vehicle is corrected so as to be the same as road data. This is called a map matching technique as disclosed in the official gazette of Japanese Patent Application Publication (KOKOKU) No. 13972/1994. This map matching technique can enhance an accuracy in calculating a current position.
Meanwhile, the diameter of a tire, which relates to a distance factor, varies continuously while driving, due to such factors as wearing away of the tire, or expansion caused by a temperature change. As a result, an error is generated in calculating the travelled distance and a highly accurate calculation of the current position cannot be performed. For example, one percent of error in the travelled distance factor for each revolution of the tire will result in an error of 1 kilometer if the travelled distance is 100 kilometers.
Such an error in measuring the travelled distance as described above can be corrected to a certain extent by using the aforementioned map matching technique if it is applied to driving on an ordinary road. However, in the event of driving on a highway or the like, particular features which can be utilized in the map matching technique are rarely encountered, such as sharp curves or intersections, and thus a correction of errors cannot be adequately performed.
Moreover, once an error of approximately 1 kilometer is produced between the estimated current position and the actual current position, it will be difficult to correct the position according to the map matching technique.
So far, the following techniques have been employed for avoiding errors in measuring a travelled distance:
(1) correcting a distance factor per revolution of a tire by comparing the road data from the point of turning at an intersection (a starting point) to the point of turning at the next intersection (a terminal point), with the travelled distance obtained by measuring the number of revolutions of a transmission output shaft or a tire; (2) correcting the aforementioned distance factor by comparing the distance between two beacons on a map, with the distance measured by driving as disclosed in the official gazette of Japanese Patent Application Publication (KOKOKU) No. 27652/1994; and (3) correcting the aforementioned distance factor by obtaining a vehicle speed from the GPS (Global Positioning System) receiving device for calculating a current position using the signals from the GPS satellites and comparing the obtained vehicle speed with a detected number of revolutions of a tire, as disclosed in the official gazette of Japanese Patent Application Laid-open (KOKAI) No. 107958/1990.